Current mode 3-state buck-boost pwm control architecture

ABSTRACT

A voltage regulator includes a converter and a modulator. The converter includes a switching circuit coupled to an inductor for converting an input voltage to an output voltage. The modulator controls the switching circuit in a buck mode of operation, a boost mode of operation, and an intermediate buck-boost mode of operation. During the buck-boost mode of operation, the modulator controls the switching circuit during each switching cycle to sequentially switch between three different switching states, including a first switching state that applies the input voltage across the inductor, a second switching state that applies a difference between the input and output voltages across the inductor, and a third switching state that applies the output voltage across the inductor. The modulator is controlled based on voltage applied across or current flowing through the inductor to regulate the output voltage to a target level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/402,048, filed Jan. 9, 2017, now U.S. Pat. No. 10,038,382. Thisapplication also claims the benefit of U.S. Provisional Application Ser.No. 62/280,577, filed on Jan. 19, 2016, U.S. Provisional ApplicationSer. No. 62/335,523, filed on May 12, 2016 and U.S. ProvisionalApplication Ser. No. 62/339,497, filed on May 20, 2016, which are herebyincorporated by reference in their entireties for all intents andpurposes.

BRIEF DESCRIPTION OF THE DRAWINGS

The benefits, features, and advantages of the present invention willbecome better understood with regard to the following description andaccompanying drawings, in which:

FIG. 1 is a simplified block diagram of an electronic device configuredwith a power system with a voltage regulator including a buck-boostmodulator implemented according to an embodiment of the presentinvention;

FIG. 2 is a simplified schematic and block diagram of the voltageregulator of FIG. 1 according to one embodiment configured for buck,boost and buck-boost operation;

FIG. 3 is a schematic diagram of a buck-boost modulator according to oneembodiment which may be used as the buck-boost modulator of FIG. 2;

FIG. 4 is a series of timing diagrams illustrating operation of thebuck-boost modulator of FIG. 3 according to one embodiment when usedwithin the buck-boost modulator of FIG. 2 during the buck-boostoperating mode;

FIG. 5 is a schematic diagram of a buck-boost modulator according toanother embodiment which may also be used as the buck-boost modulator ofFIG. 2;

FIG. 6 is a series of timing diagrams illustrating operation of thebuck-boost modulator of FIG. 5 according to one embodiment when usedwithin the buck-boost modulator of FIG. 2 during the buck-boostoperating mode; and

FIG. 7 is a timing diagram illustrating 2-state and 3-stateconfigurations for the buck-boost mode according to embodiments.

DETAILED DESCRIPTION

Voltage regulators that convert an input voltage to a regulated outputvoltage operate in buck mode when the input voltage is greater than theoutput voltage, and operate in boost mode when the input voltage is lessthan the output voltage. When the input voltage may range from less thanto greater than the output voltage, the voltage regulator must supportboth modes and must switch between the modes. Also, many configurationssupport an intermediate buck-boost mode. The buck-boost mode may be a2-state configuration based on a single pulse width modulation (PWM)signal. The 2-state configuration during the buck-boost mode generallyoperates at a higher inductor current thereby reducing overallefficiency.

A current mode 3-State buck-boost pulse width modulation (PWM) controlarchitecture as described herein provides high efficiency and fasttransient response during the intermediate buck-boost mode of operation.

FIG. 1 is a simplified block diagram of an electronic device 100configured with a power system 101 with a voltage regulator 103including a buck-boost modulator 217 implemented according to anembodiment of the present invention. The power system 101 develops oneor more supply voltages which provide power to other system devices ofthe electronic device 100. In the illustrated embodiment, the electronicdevice 100 includes a processor 107 and a peripheral system 109, bothcoupled to receive supply voltages from the power system 101 via asupply bus 105, which includes any combination of power and/or signalconductors. In the illustrated embodiment, the peripheral system 109 mayinclude any combination of a system memory 111, such as any combinationof random-access memory (RAM) and/or read-only memory (ROM) type devicesand memory controllers and the like, and an input/output (I/O) system113, which may include system and peripheral controllers and the like,such as graphic controllers, interrupt controllers, keyboard and mousecontrollers, system storage device controllers (e.g., controllers forhard disk drives and the like), etc. The illustrated system is exemplaryonly, since many of the processor system and support devices may beintegrated onto the processor chip as understood by those skilled in theart.

The electronic device 100 may be any type of computer or computingdevice, such as a computer system (e.g., notebook computer, desktopcomputer, netbook computer, etc.), a media tablet device (e.g., iPad byApple Inc., Kindle by Amazon.com, Inc., etc.), a communication device(e.g., cellular phone, smartphone, etc.), among other types ofelectronic devices (e.g., media player, recording device, etc.).

FIG. 2 is a simplified schematic and block diagram of the voltageregulator 103 according to one embodiment configured for buck, boost andbuck-boost operation. An input source 201 is coupled between an inputnode 202 and a reference source voltage node. The reference sourcevoltage node develops a reference source voltage, which may have anypositive, zero, or negative voltage level. In the illustratedembodiment, the reference source voltage is ground (GND) but may bereferenced by other names, such as VSS or the like. The input source 201develops an input voltage VIN on the input node 202 and sources an inputcurrent IIN. The input source 201 may be implemented as an AC-DC adapterand/or a battery in which a battery charger or the like may also beincluded. An input capacitor 203 with capacitance CI is coupled betweenthe input node 202 and GND. A buck power stage 205 is coupled betweenthe input node 202 and GND. The buck power stage 205 includes a firstelectronic switch Q1 having current terminals coupled between the inputnode 202 and a first intermediate node 206, and a second electronicswitch Q2 having current terminals coupled between node 206 and GND. Aninductor 207 with inductance LO is coupled between the firstintermediate node 206 and a second intermediate node 208. An inductorcurrent IL is shown flowing from node 206 to node 208 through theinductor 207. A boost power stage 209 is coupled between an output node210 and GND in which the output node develops an output voltage VO. Theboost power stage 209 includes a first electronic switch Q4 havingcurrent terminals coupled between the output node 210 and the secondintermediate node 208, and a second electronic switch Q3 having currentterminals coupled between node 208 and GND.

An output capacitor 211 with capacitance CO and a load 213 drawing anoutput load current 10 are both coupled between the output node 210 andGND. The load 213 may represent any combination of the system devicesdrawing load current, such as the processor 107, the system memory 111,the peripheral system 109, etc. The output voltage VO is providedthrough a VO sense block 214 to provide an output voltage sense signalVS provided to an input of an error amplifier and compensation (EA ANDCOMP) block 215. The VO sense block 214 may simply be a buffer orconductor or the like in which VS has the same voltage level as VO.Alternatively, or additionally, the VO sense block 214 may include avoltage divider or the like in which the voltage level of VS follows VOat a proportional level. In either event, VS represents VO as a sensedversion thereof. The error amplifier and compensation block 215 alsoreceives a reference voltage VREF at another input, and provides acompensation or control voltage VC at its output. VREF has a voltagelevel indicative of a target level of VO in which the control loopoperates to keep VS at about the level of VREF. VC is provided to aninput of the buck-boost modulator 217, which also receives or detectsthe input voltage VIN and the output voltage VO. The buck-boostmodulator 217 has a first output providing a first pulse control signalPWM_BUCK to a first driver (DRV) 219 for driving the gates of Q1 and Q2,and has a second output providing a second pulse control signalPWM_BOOST to a second driver 221 for driving the gates of Q3 and Q4.

Each of the drivers 219 and 221 are configured to convert thecorresponding pulse control or PWM signal (PWM_BUCK or PWM_BOOST) intothe gate drive signals for turning on and off the corresponding switchesQ1 and Q2 or Q3 and Q4. As an example, in a buck mode of operation, thePWM_BOOST signal may be held at a fixed value to keep Q4 on and Q3 offso that the output end of the inductor 207 remains coupled to the outputnode 210. During the buck mode of operation, when PWM_BUCK is at onestate (e.g., high), the driver 219 turns on Q1 and turns off Q2, andwhen PWM_BUCK is at another or opposite state (e.g., low), the driver219 turns off Q1 and turns on Q2. The driver 219 may be configured toensure that Q1 and Q2 are not turned on at the same time. In the boostmode of operation, PWM_BUCK may be held high so that Q1 remains on andQ2 remains off, and PWM_BOOST toggles during successive cycles so thatthe driver 221 toggles activation of Q3 and Q4 in similar manner toeffectuate boost operation. For example, when PWM_BOOST is high, Q4 isturned on while Q3 is turned off, and when PWM_BOOST goes low, Q4 isturned off and Q3 is turned on. The driver 221 may also be configured toensure that Q3 and Q4 are not turned on at the same time.

The error amplifier and compensation block 215 drives VC based on adifference between VS (representative of VO) and VREF. As an example, anerror amplifier (not shown) may have a first input receiving VS and asecond input receiving VREF and an output providing VC. A compensationcircuit, such as a resistor-capacitor (RC) circuit or the like, may beincluded to provide loop compensation as understood by those of ordinaryskill in the art. The voltage regulator 103 is configured as ahysteretic current mode buck-boost PWM control system as controlled bythe buck-boost modulator 217 as further described herein.

The electronic switches Q1-Q4 are each shown as MOS or FET typetransistors, such as a P-type MOS (PMOS) or P-type FET (PFET)transistor, an N-type MOS (NMOS) or P-type FET (PFET) transistor, or aMOSFET transistor or the like. Alternative power switching devices orelectronic control switches are contemplated, such as bipolar junctiontransistors (BJTs) and the like, insulated-gate bipolar transistors(IGBTs) and the like, etc. Each switch device is sized and configured toperform the intended function. The switches Q1-Q4 and the inductor 207are collectively part of a converter 225 that is controlled by thebuck-boost modulator 217 to convert the input voltage VIN to the outputvoltage VO.

The voltage regulator 103 is shown in simplified form in which manydetails are omitted as not relevant for a complete understanding of thepresent invention. Also, the present disclosure is directed towardsoperation in the intermediate buck-boost mode of operation rather thanthe buck mode or the boost mode. The buck-boost modulator 217 determinesthe mode of operation and generates the PWM_BUCK and PWM_BOOST signalsaccordingly based on the mode of operation. As previously described,during the buck mode when VIN is greater than VO, the PWM_BOOST signalmay be held high to turn Q4 on and Q3 off while PWM_BUCK oscillates totoggle activation of Q1 and Q2. Similarly, during the boost mode whenVIN is less than VO, the PWM_BUCK signal may be held high to turn Q1 onand Q2 off while PWM_BOOST oscillates to toggle activation of Q3 and Q4.In one embodiment, the determination of the mode of operation is made bycomparing VIN with VO. Although VO may be varied in some embodiments, VOis typically regulated to a predetermined target voltage level asdetermined by the voltage level of VREF. VIN, on the other hand, mayhave a relatively wide range from below VO to above VO. When VIN and VOare separated from each other by a significant amount, then one of thebuck or boost operating modes is selected. When VIN is relatively closeto VO, such as within a predetermined percentage of VO (e.g., 12%), thenthe buck-boost operating mode is selected.

A conventional configuration uses a 2-state buck-boost operation inwhich operation toggles between two states during each PWM cycle. In thefirst switching state, Q1 and Q3 are turned on while Q2 and Q4 areturned off. As shown by the voltage regulator 103, when Q1 and Q3 areturned on, VIN is applied to a first end of the inductor 207 at node 206and GND is applied to the second end at node 208. The inductor currentIL ramps up during the first switching state. In the second switchingstate of the 2-state buck-boost configuration, Q2 and Q4 are turned onwhile Q1 and Q3 are turned off. When Q2 and Q4 are turned on, VO isapplied to the second end of the inductor 207 at node 208 and GND isapplied to the first end at node 206. The inductor current IL ramps downduring the second switching state since the voltage applied across theinductor 207 is effectively reversed. In other words, the voltage VIN isapplied in the first state and the voltage −VO is applied in the secondstate. Operation toggles between these two states during each switchingcycle in the conventional 2-state buck-boost configuration.

In the 3-state buck-boost operation as described herein, an intermediatestate is interposed between the first and second switching states of theconventional configuration for each switching cycle. The first switchingstate of the 3-state buck-boost configuration, referred to as theswitching state S1, is similar to that of the conventional configurationin which Q1 and Q3 are turned on while Q2 and Q4 are turned off so thatthe voltage VIN is applied across the inductor 207 and the inductorcurrent IL ramps up. The second switching state of the 3-statebuck-boost configuration as described herein, however, is different fromthat of the conventional 2-state buck-boost configuration. In the secondswitching state, referred to as the switching state S2, Q1 and Q4 areturned on while Q2 and Q3 are turned off so that VIN is applied to thefirst end of the inductor 207 at node 206 and VO is applied to thesecond end of the inductor 207 at node 208. In this manner, the voltageVIN−VO is applied across the inductor 207, which is the differencebetween the input and output voltages. Since VIN and VO are relativelyclose or even about equal to each other, the inductor current ILflattens when VIN=VO, or slowly ramps up when VIN>VO, or slowly rampsdown when VIN<VO. The third state of 3-state buck-boost configuration,referred to as S3, is similar to the second state of the conventional2-state configuration, in which Q2 and Q4 are turned on while Q1 and Q3are turned off. In the third switching state S3 of the 3-statebuck-boost configuration as described herein, the voltage −VO is appliedacross the inductor 207 so that the inductor current IL ramps back down.

The decision for switching between the three switching states S1, S2 andS3 during each switching cycle is further described herein. In contrastto the conventional configuration, 3-state buck-boost operation adds theintermediate second state by introducing a controlled delay on thefalling edge of a PWM signal to force an intermediate VIN−VO statebetween the VIN (S1) and −VO (S3) states. Since the inductor 207 isconnected to the output voltage VO for a longer period of time for the3-state configuration in each cycle, the average inductor current ismuch lower and efficiency is improved. The intermediate switching stateS2 may be a programmable percentage of the overall switching period(K*ts), in which “K” is a programmable gain constant and an asterisk “*”denotes multiplication. The 3-state control scheme ensures fasttransient response and current-mode stability.

FIG. 3 is a schematic diagram of a buck-boost modulator 300 according toone embodiment which may be used as the buck-boost modulator 217. Thebuck-boost modulator 300 includes a synthetic current ramp generator301, a PWM comparator 303, a ramp generator and PWM comparator 305 and amode comparator 307. The synthetic current ramp generator 301 includes acurrent source 311, switches 313 and 315, a current sink 317 and a rampcapacitor 319 having a capacitance CR1. The current source 311 an theswitch 313 are coupled in series between a source voltage (e.g., VDD)and a ramp node 314 developing a ramp voltage VR1. The switch 315 andthe current sink 317 are coupled in series between the ramp node 314 andGND. The ramp capacitor 319 is coupled between the ramp node 314 andGND.

The current source 311 sources a current gm*VIN to a ramp node 314 tocharge the ramp capacitor 319 when the switch 313 is closed by a signalPWM2. The current sink 317 selectively pulls a current gm*VO from theramp node 314 to GND to discharge the ramp capacitor 319 when the switch315 is closed by a signal PWM1 . The signal PWM1 is an inverted versionof a signal PWM1. The current gm*VIN is proportional to the inputvoltage VIN by a gain factor gm, and the current gm*VO is proportionalto the output voltage VO by the gain factor gm. The gain factor gm is atransconductance gain factor, in which the current source 311 andcurrent sink 317 may each be implemented by a transconductance amplifieror the like.

In operation of the synthetic current ramp generator 301, when PWM2 isasserted high to close switch 313 and PWM1 is asserted low (PWM1asserted high) to open switch 315, then the capacitor 319 is chargedwith the current gm*VIN to ramp VR1 up proportional to the input voltageVIN. When PWM2 is asserted low to open the switch 313 and PWM1 isasserted high (PWM1 asserted low) to close switch 315, then thecapacitor 319 is discharged with the current gm*VO to ramp VR1 downproportional to the output voltage VO (thus VR1 ramps down with −VO).When PWM2 and PWM1 are both asserted high, then VR1 ramps proportionalto VIN −VO.

The PWM comparator 303 includes window current sources 321 and 323 eachdeveloping a window current IW, window resistors 325 and 327 each havinga resistance RW/2 (each half of a window resistance RW), comparators 329and 331, and a set-reset (SR) latch 333. The current sources 321 and 323and the window resistors 325 and 327 are coupled in series between asource voltage (e.g., VDD) and GND having a center node 328 receivingthe control voltage VC. The window resistor 325 is coupled between thecenter node 328 and an upper window node 322 which develops an upperwindow voltage VWP1 provided to the negative input (or inverting input)of the comparator 329. The window resistor 327 is coupled between node328 and a lower window node 326 which develops a lower window voltageVWN1 provided to the positive input (or non-inverting input) of thecomparator 331. As VC varies based on load conditions and otheroperating conditions, VWP1 and VWN1 follow above and below,respectively, the voltage of VC. Since the resistances (RW/2) of theresistors 325 and 327 and the currents (IW) of the current sources 321and 323 are about equal, VWP1 remains above VC by a half a windowvoltage VW, or VW/2, whereas VWN1 remains VW/2 below the voltage of VCand the window voltage VW=VWP1−VWN1. VR1 is provided to the positiveinput of the comparator 329 and to the negative input of the comparator331. The output of the comparator 329 provides a reset signal R to areset input of the SR latch 333. The output of the comparator 331provides a set signal S to the set input of the SR latch 333. The Qoutput of the SR latch 333 asserts the PWM1 signal.

In operation of the PWM comparator 303, when VR1 falls at or just belowthe voltage of VWN1, the comparator 331 asserts the S signal to set theSR latch 333 to assert PWM1 high. When VR1 rises to or just above thevoltage of VWP1, the comparator 329 asserts the R signal to reset the SRlatch 333 to assert PWM1 low. As noted previously, the PWM1 signal is aninverted version of PWM1 for controlling the switch 315.

The ramp generator and PWM comparator 305 includes current sources 341and 343, a switch 345 controlled by the PWM1 signal, a capacitor 347with capacitance CR2 coupled between another ramp node 342 and GND, aresistor 349 with resistance RW, a comparator 351 and a 2-input Booleanlogic OR gate 353. The ramp node 342 is provided to the negative inputof the comparator 351. The ramp generator and PWM comparator 305 mayalso include a buffer 355 and an inverter 357. In one embodiment, thecapacitance CR2 is the same as CR1, or CR1=CR2. The output of thecurrent source 343 is coupled to one end of the resistor 349, having itsother end coupled to GND. The current source 343 develops a currentgm*VIN*K through the resistor 349 developing a voltage VWP2 provided tothe positive input of the comparator 351. Thus, the voltage VWP2 isproportional to the input voltage VIN by the gain factor gm*K. Thecurrent source 341 develops the current gm*VIN provided to the ramp node342 to charge the ramp capacitor 347 when the switch 345 is open. Thevoltage of the capacitor 347 is the voltage of the ramp node 342, whichis shown as a second ramp voltage VR2. The switch 345 is coupled betweennode 342 and GND. When the switch 345 is closed, the capacitor 347 isshorted to GND so that VR2 is pulled to GND. The output of thecomparator 351 is provided to a first input of the OR gate 353, whichreceives PMW1 at its other input and which provides the PWM2 signal atits output.

In operation of the ramp generator and PWM comparator 305, when PWM1 isasserted high by the PWM comparator 303, the PWM2 signal is alsoasserted high by the OR gate 353. Also, the switch 345 is closed so thatVR2 is pulled lower than VWP2 so that the comparator 351 asserts itsoutput high. When PWM1 goes low, PWM2 remains high since the output ofthe comparator 351 is still high. Also, when PWM1 goes low the switch345 is opened so that the capacitor 347 is charged by the current source341 at a rate proportional to the input voltage VIN. When VR2 rises toor just above VWP2, the comparator 351 asserts its output low so thatthe OR gate 353 pulls PWM2 low. When PWM1 next goes high, PWM2 is alsopulled high by the OR gate 353 and operation repeats in this manner insuccessive cycles. The buffer 355 receives PWM2 at its input andprovides PWM_BUCK at its output, so that the PWM_BUCK signal may be abuffered version of PWM2. Alternatively, PWM2 may be used directly asthe PWM_BUCK signal. The inverter 357 receives PWM1 at its input andprovides PWM_BOOST at its output, so that PWM_BOOST is an invertedversion of PWM1.

The mode comparator 307 receives the input and output voltages VIN andVO and determines the mode of operation between the buck operating mode(BUCK), the boost operating mode (BOOST), or the buck-boost operatingmode (BUCK_BOOST). In the illustrated embodiment, the transition betweenoperating modes occurs when VIN is within about 12% of VO. VO isprovided to the input of a first gain block 360 that outputs a voltage0.88*VO that is about 12% less than VO, in which 0.88*VO is provided tothe positive input of a boost comparator 362. The boost comparator 362receives VIN at its negative input and outputs a logic signal BOOST.BOOST is high indicating the boost operating mode when VIN is less thanabout 88% the voltage level of VO, but is otherwise low. VIN is providedto the input of a second gain block 364 that outputs a voltage 0.893*VINthat is about 12% greater than VO. It is noted that when VIN is about12% higher than VO, that VO is about 89.3% of VIN (1/1.12≈0.893). Thevalue 0.893*VIN is provided to the positive input of a boost comparator366, which receives VO at its negative input and outputs a logic signalBUCK. BUCK is high indicating the buck operating mode when VO is lessthan about 89.3% of the voltage level of VIN, but it otherwise assertedlow. The BOOST and BUCK logic signals are provided to a BOOLEAN logicNOR gate 368, which outputs a BUCK_BOOST logic signal indicating thebuck-boost operating mode when asserted high. Generally, when VIN iswithin about 12% of VO, then BUCK and BOOST are both low so thatBUCK_BOOST is high indicating the buck-boost operating mode. If eitherBUCK or BOOST is asserted high, then BUCK_BOOST is low.

FIG. 4 is a series of timing diagrams illustrating operation of thebuck-boost modulator 300 according to one embodiment when used withinthe buck-boost modulator 217 of FIG. 2 during the buck-boost operatingmode. An upper timing diagram plots the voltages VWP1, VWN1, VR1 and VCtogether versus time. A middle timing diagram plots the voltages VR2 andVWP2 together versus time. A lower timing diagram plots PWM1, PWM2, andcorresponding PWM signals PWM_BUCK and PWM_BOOST versus time. The pulsecontrol signals PWM1 and PWM2 generally define the switching state inwhich the buck-boost modulator 300 sequentially transitions between thestates S1-S3 as shown. The PWM_BUCK and PWM_BOOST signals are used tocontrol switching of the buck power stage 205 and the boost power stage209, respectively, of the converter 225. PWM_BUCK is shown having thesame states and state transitions as PWM2, and PWM_BOOST is shown as aninverted version of PWM1. At an initial time t0, PWM1 and PWM2 both gohigh initiating the first switching state S1. At next time t2, PWM1 goeslow so that the switching state transitions from S1 to S2. When PWM2next goes low at time t2, the switching state transitions from S2 to S3.When PWM1 and PWM2 both go high at subsequent time t3, the switchingstate transitions from S3 back to S1. Operation repeats in this manner.

As shown in FIG. 4, VWP1 remains at a constant half window voltage aboveVC, and WWN1 remains at a constant half window voltage below VC. VC isshown at a steady value until it rises in response to a load transient,and then falls in response to a release of the load transient. VWP1 andVWN1 remain displaced from VC based on the window voltage as shown. PWM1and PWM2 are both asserted high at time t0 initiating the switchingstate S1, so that and VR1 rises from VWN1 towards VWP1. VR1 risesproportional to the input voltage VIN. When VR1 reaches VWP1 at time t1,PWM1 is pulled low transitioning the switching state to S2, VR1 flattensand ramps proportional to VIN−VO, and VR2 begins ramping from GNDtowards VWP2 during the switching state S2. VR1 is shown at a flat ratewhile ramping proportional to VIN−VO, which occurs when VIN and VO arerelative close to each other, where it is understood that the slope oframping is based on the difference between VIN and VO. PWM2 remains highwhile VR2 is less than VWP2. When VR2 reaches VWP2 at time t2, PWM2 ispulled back low transitioning the switching state to S3, and VR1 rampsdown proportional to −VO during switching state S3. Since PWM1 is stilllow, VR2 continues to ramp up. When VR1 falls to (or just below) VWN1 attime t3, PWM1 and PWM2 are both asserted high again transitioning theswitching state back to S1, VR2 is shorted back to GND, and VR1 beginsramping back up once again from VWN1 towards VWP1 during the switchingstate S1 between times t3 and t4. When VR1 reaches VWP1 at time t4,operation transitions to switching state S2, and when VR2 reaches VWP2at time t5, operation transitions to switching state S3.

During operation, VC rises in response to a load transient, stays highfor a period, and then goes back low in response to release of the loadtransient. In the simplified illustration, VC starts to rise just as VR2reaches VWP2 at time t5, where it is understood that VC may start torise at any asynchronous time during the cycle. Also, when VC rises,VWN1 and VWP1 rise accordingly as controlled by the window circuit(current sources 321 and 323 and resistors 322 and 327). When VR2reaches VWP2 at time t5, PWM2 goes low and VR1 begins ramping down whileVR2 continues to rise during the switching state S3. When VR1 falls toVWN1 (which is still rising) at time t6, PWM1 and PWM2 both go high andoperation transitions back to switching state S1. VR1 rises at a rateproportional to VIN (not necessarily the same rate as VC or VWN1), andVR2 is clamped low to GND during S1. When VR1 reaches VWP1 at time t7,PWM1 goes low to transition to switching state S2, so that VR1 rampsproportional to VIN−VO and VR2 is released to ramp up at a rateproportional to VIN. When VR2 eventually reaches VWP2 at time t8, PWM2is pulled low to transition to the switching state S3, so that VR1 rampsdown proportional to −VO while VR2 continues to rise during S3. At thistime, VC and VWN1 have both ramped down to a relatively low level. Thus,VR1 ramps from a relatively high state all the way to VWN1 while VR2continues to rise. When VR1 eventually falls to VWN1 at time t9, PMW1and PMW2 both go high again to transition to the switching state S1, andoperation repeats in this manner during the buck-boost mode ofoperation.

The PWM_BUCK signal essentially follows PWM2, meaning that they may bethe same signal or PWM_BUCK may be a buffered version of PWM2. ThePWM_BOOST essentially follows as the PWM1 , meaning that they may be thesame signal or PWM_BOOST may be a buffered version of PWM1 or aninverted version of PWM1. One observation of the operation shown in FIG.4 is that when VC falls lower, such as in response to a release of theload transient, VR1 stays relatively high while VR2 ramps up past VWP2.VR2 continues to ramp up while VR1 ramps down, so that the cycle doesnot end until VR1 reaches VWN1. As described further below, it ispossible to shorten this portion of the extended cycle.

FIG. 5 is a schematic diagram of a buck-boost modulator 500 according toanother embodiment which may also be used as the buck-boost modulator217. The buck-boost modulator 500 is substantially similar to thebuck-boost modulator 300, and includes the synthetic current rampgenerator 301, the PWM comparator 303 and the mode comparator 307 thatare each configured to operate in substantially the same manner. Theramp generator and PWM comparator 305, however, is replaced by a similarramp generator and PWM comparator 505. The ramp generator and PWMcomparator 505 also includes the same components coupled insubstantially the same manner, namely, the current sources 341 and 343,the switch 345 controlled by the PWM1 signal, the capacitor 347 coupledbetween the ramp node 342 and GND, the resistor 349 with resistance RW,the comparator 351, the OR gate 353, the buffer 355 and the inverter357.

For the ramp generator and PWM comparator 505, however, the switch 345is not coupled and referenced to GND, but instead is coupled andreferenced to the VWN1 voltage (lower window voltage). Also, theresistor 349 is not coupled and referenced to GND, but instead iscoupled and referenced to the VWN1 voltage. In this manner, the voltagesVR2 and VWP2 are not referenced to GND but are instead referenced toVWN1 and thus follow the control voltage VC. When the switch 345 isclosed, the capacitor 347 is not shorted to GND but is instead pulled tothe voltage level of VWN1. The voltage of VWP2 is the voltage of VWN1plus a voltage proportional to VIN by operation of the current source343 sourcing the current gm*VIN*K through the resistor 349 withresistance RW.

FIG. 6 is a series of timing diagrams illustrating operation of thebuck-boost modulator 500 according to one embodiment when used withinthe buck-boost modulator 217 of FIG. 2 during the buck-boost operatingmode. An upper timing diagram plots the voltages VWP1, VWN1, VR1, VC,VR2 and VWP2 together versus time. In this case, VR2 and VWP2 areplotted with the other voltages since biased relative to VWN1. A lowertiming diagram plots PWM1, PWM2, and corresponding PWM signals PWM_BUCKand PWM_BOOST versus time. Again, the PWM_BUCK and PWM_BOOST signals areused to control switching of the buck power stage 205 and the boostpower stage 209, respectively, of the converter 225, and the pulsecontrol signals PWM1 and PWM2 generally define the switching state inwhich the buck-boost modulator 500 also sequentially transitions betweenthe states S1-S3 as shown.

As shown in FIG. 6, just as with that shown in FIG. 4, VWP1 remains at aconstant half window voltage above VC, and WWN1 remains at a constanthalf window voltage below VC. VC is shown at a steady value until itrises in response to a load transient at time t5, and then falls inresponse to a release of the load transient beginning at about time t7.VWP1 and VWN1 remain displaced from VC based on the window voltage asshown. Operation is substantially similar to that shown in FIG. 4 beforeapplication of the load transient from time t0 to time t5, except thatVR2 is reference to VWN1 rather than GND, and VWP2 changes with changesof VWN1 (and thus with VC). When VR2 reaches VWP2 at time t5 causing theswitching state to transition from S2 to S3, VC ramps up in response tothe load transient.

Operation is similar when VC rises in response to application of theload transient, except that VWP2 rises with VC during the switchingstate S3, and when VR1 falls to VWN1 at time t6 transition the switchingstate to S1, VR2 falls back to VWN1. In this simplified example, VR2reaches VWP2 at about the same time VC begins rising. In this case, whenPWM1 and PWM2 are asserted at time t6, VR2 falls back to VWN1, and thenrises with VWN1 since clamped to VWN1 during the switching state S1.When VC flattens after reaching a peak level, VWN1 also flattens alongwith VR2 which is effectively clamped to VWN1 at this time. VR1continues to rise until it reaches VWP1 at time t7, and then PWM1 ispulled low to transition to the switching state S2. At about the sametime in the simplified illustration, VC, VWN1 and VWP1 begin fallingback low in response to release of the transient. However, VR2 beginsramping up at time t7 in response to PWM1 going low, while VR1 rampsproportional to VIN−VO during S2. When VR2 rises above VWP2 (which isramping down) at time t8, PWM2 goes low to transition to switching stateS3 so that VR1 begins ramping down at a rate proportional to −VO. Ascompared to that shown in FIG. 4, VR1 ramps down sooner in the cycle.VR2 continues to rise while VR1 is ramping down, until VR1 falls back toVWN1 at time t9 to cause transition to switching state S1, at which timeVR2 is pulled back to the voltage level of VWN1 and PWM1 and PM2 areasserted high to begin the next switching cycle. Operation repeats insimilar manner.

FIG. 7 is a timing diagram plotting the inductor current IL, in Amperes(A), versus the input voltage VIN in Volts (V) for the 2-state and3-state configurations for the buck-boost mode, with K=0.5 and VO=12.6V.A buck curve 701 is shown for the input voltage VIN above the outputvoltage VO with IL at about 1 A, and a boost curve 703 is shown for VINbelow VO in which IL rises as VIN falls below VO. The curves 701 and 703are shown intersecting at a point 702 when VIN=VO=12.6V. Ideally,operation would follow the curves 701 and 703 for minimized inductorcurrent IL. Regulation operation, however, has proved to be ratherdifficult when VIN is sufficiently close to VO and it is difficult totransition between the buck and boost modes of operation when VIN and VOare about equal. Instead, when VIN is about 14.3V (1.12*V0) or greater,then operation is in the buck operating mode in which the inductorcurrent IL is generally defined by the buck curve 701, which is about 1A. When the input voltage VIN is about 11.1V (0.88*VO) or less, thenoperation is in the boost operating mode in which the inductor currentis generally defined by the boost curve 703, which ranges from about 1.2A to 2.5V (for VIN=5V) during the boost mode of operation. When VIN isabove about 0.88*VO or below about 1.12*VO, then operation is in thebuck-boost operating mode.

A 2-state buck-boost curve 705 is shown applicable during the buck-boostoperating mode, in which it ranges from about 1.8 A (at the boost modetransition) to about 2.2 A (at the buck mode transition). Also, a3-state buck-boost curve 707 is shown applicable during the buck-boostoperating mode, in which it ranges from about 1.2 A (at the boost modetransition) to about 1.4 A (at the buck mode transition). In thismanner, the 3-state buck-boost curve 707 remains at about 1 A less thanthe 2-state buck-boost curve 705 during the buck-boost operating range.A dotted line 709 indicates the system operating curve over the entirerange of operation for the buck, buck-boost and boost modes of operationfor the 3-state configuration using the buck-boost modulators 300 or 500as the buck-boost modulator 217 in the voltage regulator 103 for theparticular implementation. In this manner, the efficiency of operationof the voltage regulator 103 is substantially improved since theinductor current IL is significantly reduced during the buck-boostoperating mode.

The benefits, features, and advantages of the present invention are nowbetter understood with regard to the foregoing description andaccompanying drawings. The foregoing description was presented to enableone of ordinary skill in the art to make and use the present inventionas provided within the context of a particular application and itsrequirements. Various modifications to the preferred embodiment will,however, be apparent to one skilled in the art, and the generalprinciples defined herein may be applied to other embodiments.Therefore, the present invention is not intended to be limited to theparticular embodiments shown and described herein, but is to be accordedthe widest scope consistent with the principles and novel featuresherein disclosed. Although the present invention has been described inconsiderable detail with reference to certain preferred versionsthereof, other versions and variations are possible and contemplated.Those skilled in the art should appreciate that they can readily use thedisclosed conception and specific embodiments as a basis for designingor modifying other structures for providing the same purposes of thepresent invention without departing from the spirit and scope of theinvention as defined by the following claim(s).

What is claimed is:
 1. A voltage regulator, comprising: a convertercomprising a switching circuit coupled to an inductor for converting aninput voltage to an output voltage; and a modulator that controls saidswitching circuit in a buck mode of operation, a boost mode ofoperation, and an intermediate buck-boost mode of operation, whereinduring said buck-boost mode of operation, said modulator controls saidswitching circuit during each switching cycle to sequentially switchbetween three different switching states, comprising: a first switchingstate that applies said input voltage across said inductor; a secondswitching state that applies a difference between said input and outputvoltages across said inductor; and a third switching state that appliessaid output voltage across said inductor, wherein the modulator controlsa duration of the second switching state by generating a time delay atthe end of the first switching state, and wherein the second switchingstate ends and the third switching state begins after the time delay. 2.The voltage regulator of claim 1, wherein said modulator is operativeduring said buck-boost mode of operation to control said switchingcircuit to: couple a first end of said inductor to said input voltageand couple a second end of said inductor to a reference source voltageduring said first switching state; couple said first end of saidinductor to said input voltage and couple said second end of saidinductor to said output voltage during said second switching state; andcouple said first end of said inductor to said reference source voltageand couple said second end of said inductor to said output voltageduring said third switching state.
 3. The voltage regulator of claim 1,wherein: said switching circuit comprises: a first switch coupledbetween said input voltage and a first end of said inductor; a secondswitch coupled between said first end of said inductor and a referencesource voltage node; a third switch coupled between a second end of saidinductor and said reference source voltage node; and a fourth switchcoupled between said output voltage and said second end of saidinductor; and wherein said modulator is operative during said buck-boostmode of operation to: turn on said first and third switches and turn offsaid second and fourth switches during said first switching state; turnon said first and fourth switches and turn off said second and thirdswitches during said second switching state; and turn on said second andfourth switches and turn off said first and third switches during saidthird switching state.
 4. The voltage regulator of claim 1, furthercomprising a comparator system that compares said input voltage withsaid output voltage and that selects said buck-boost mode of operationwhen said input voltage is within a predetermined percentage of saidoutput voltage, that selects said buck mode of operation when said inputvoltage is greater than said output voltage by more than saidpredetermined percentage of said output voltage, and that selects saidboost mode of operation when said input voltage is less than said outputvoltage by more than said predetermined percentage of said outputvoltage.
 5. The voltage regulator of claim 1, wherein said modulatorcontrols said second switching state by causing the duration to be apredetermined percentage of said each switching cycle.
 6. The voltageregulator of claim 1, wherein said modulator controls said secondswitching state by causing the duration to be a programmable percentageof said each switching cycle.
 7. The voltage regulator of claim 1,further comprising: an error amplifier circuit that develops amodulation control voltage based on comparing a voltage indicative ofsaid output voltage with an output reference indicative of a targetlevel of said output voltage; wherein said switching circuit furthercomprises: a buck power stage that selectively couples a first end ofsaid inductor either to said input voltage or to a reference sourcevoltage as controlled by a buck pulse control signal; and a boost powerstage that selectively couples a second end of said inductor either tosaid output voltage or to said reference source voltage as controlled bya boost pulse control signal; and wherein said modulator uses said inputvoltage, said output voltage and said modulation control voltage togenerate said buck pulse control signal and said boost pulse controlsignal.
 8. The voltage regulator of claim 1, wherein said modulatortransitions between said first, second and third switching states basedon voltage applied across said inductor.
 9. The voltage regulator ofclaim 1, wherein said modulator transitions between said first, secondand third switching states based on current through said inductor.
 10. Amethod of converting an input voltage to an output voltage, comprising:operating a converter with an inductor in a selected one of a buckoperating mode, a boost operating mode, and a buck-boost operating mode;and during the buck-boost operating mode for each switching cycle,operating the converter to sequentially switch between first, second andthird switching states, comprising: applying the input voltage acrossthe inductor during the first switching state; applying a differencebetween the input and output voltages across the inductor during thesecond switching state; controlling a duration of the second switchingstate by generating a time delay at the end of the first switchingstate, wherein the second switching state ends and the third switchingstate begins after the time delay; and applying the output voltageacross the inductor during the third switching state.
 11. The method ofclaim 10, wherein said operating the converter during the buck-boostoperating mode comprises: coupling the first end of the inductor to theinput voltage and coupling the second end of the inductor to thereference source voltage during the first switching state; coupling thefirst end of the inductor to the input voltage and coupling the secondend of the inductor to the output voltage during the second switchingstate; and coupling the first end of the inductor to the referencesource voltage and coupling the second end of the inductor to the outputvoltage during the third switching state.
 12. The method of claim 10,further comprising: comparing the input voltage with the output voltage;selecting the buck-boost operating mode when the input voltage is withina predetermined percentage of the output voltage; selecting the buckoperating mode when the input voltage is greater than the output voltageby more than the predetermined percentage of the output voltage; andselecting the boost operating mode when said input voltage is less thansaid output voltage by more than said predetermined percentage of saidoutput voltage.
 13. The method of claim 10, wherein said operating theconverter during the buck-boost operating mode comprises operating theconverter in the second switching state for the duration that is aprogrammable percentage of each switching cycle.
 14. The method of claim10, wherein said operating the converter to sequentially switch betweenfirst, second and third switching states comprises operating theconverter to sequentially switch between first, second and thirdswitching states based on voltage applied across the inductor.
 15. Themethod of claim 10, wherein said operating the converter to sequentiallyswitch between first, second and third switching states comprisesoperating the converter to sequentially switch between first, second andthird switching states based on current through the inductor.
 16. Avoltage regulator, comprising: a converter comprising a switchingcircuit coupled to an inductor for converting an input voltage to anoutput voltage; a modulator that controls said switching circuit in abuck mode of operation, a boost mode of operation, and an intermediatebuck-boost mode of operation, wherein during said buck-boost mode ofoperation, said modulator controls said switching circuit during eachswitching cycle to sequentially switch between three different switchingstates; and an error amplifier circuit that develops a modulationcontrol voltage based on comparing a voltage indicative of said outputvoltage with an output reference indicative of a target level of saidoutput voltage, wherein said switching circuit comprises: a buck powerstage that selectively couples a first end of said inductor either tosaid input voltage or to a reference source voltage as controlled by abuck pulse control signal; and a boost power stage that selectivelycouples a second end of said inductor either to said output voltage orto said reference source voltage as controlled by a boost pulse controlsignal, and wherein said modulator uses said input voltage, said outputvoltage and said modulation control voltage to generate said buck pulsecontrol signal and said boost pulse control signal, and wherein saidmodulator comprises: a ramp generator that develops a ramp controlvoltage based on said input voltage, said output voltage, a first pulsecontrol signal and a second pulse control signal; a comparator circuitthat compares said modulation control voltage with said ramp controlvoltage for developing said first pulse control signal; a controlleddelay circuit that develops said second pulse control signal based onsaid first pulse control signal by transitioning said second pulsecontrol signal to a first state when said first pulse control signal istransitioned to said first state, and by transitioning said second pulsecontrol signal to a second state after a programmed delay period aftersaid first pulse control signal has transitioned to said second state;and a pulse control system that develops said buck pulse control signalas a buffered version of said second pulse control signal and thatdevelops said boost pulse control signal as an inverted version of saidfirst pulse control signal.